Method of making a fiber optic array

ABSTRACT

A method is disclosed for making a fiber optic array for use in an optical scanning device. The array includes a substrate and a plurality of optical fibers arranged on the substrate to form a linear array of the fibers. Each of the fibers is received in grooves formed in the substrate to precisely space the fibers relative to each other. Each fiber is adapted to receive light from a source such as a laser diode. In order to increase the packing density of the fibers in the array, the fibers are closely spaced relative to each other on the substrate. The fibers are arranged in aligned sets of grooves which are spaced apart by planar areas on the substrate, and the pitch of the grooves in successive sets decreases as the fibers approach an output end of the array to draw the fibers closer together at this end.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned U.S. patent applications, Ser.No. 254,757, entitled Fiber Optic Array, filed in the name of Kaukeinenon even date herewith, Ser. No. 254,759, entitled Fiber Optic Array,filed in the names of Hsu, Owens, and Sarraf, on even date herewith, andSer. No. 254,756, entitled Method of Making a Fiber Optic Array, filedon in the names of Hsu, Rao, and Kaukeinen on even date herewith.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of making a fiber optic array,and more particularly, to a method of making such an array which resultsin an improved packing density of the optical fibers.

2. State of the Prior Art

It is known in the prior art to use an array of optical fibers in aprint head which is used to record information on a light-sensitiverecording medium. The fibers can be arranged with their output ends in alinear array extending in a direction transverse to the direction ofmovement of the recording medium, and a light source, such as alight-emitting diode or a diode laser, can be connected to each of thefibers at an input end. The light in each of the fibers is modulated inaccordance with an information signal to produce a desired image.Focusing means can be used in front of each fiber to cause the light tobe focused to a point on the recording medium. It is desirable for thearrays of optical fibers to have a high packing density, i.e., a highnumber of fibers per unit width of the array, in order to limit theamount of data buffering needed to produce the output image. There is aproblem, however, in using increasingly thinner fibers to increase thepacking density. As the fibers are made thinner, handling and alignmentof the fibers becomes more difficult, and the thinner fibers are morelikely to break in the assembly process.

In U.S. Pat. No. 4,389,655, there is shown an optical device fornon-impact recording in which the recording head includes a linear arrayof optical fibers. The recording head comprises an adjustment platehaving a plurality of grooves therein, and an optical fiber is securedin each of the grooves. In one embodiment of the invention, the fibershave been arranged in grooves which converge toward the output end ofthe array to closely space the output ends of the fibers. A problem withthis arrangement is that the output ends of the fibers are not disposedat a right angle to the end of the substrate, and thus, correctiveoptics must be used to obtain a optimum exposure pattern on a recordingmedium.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the problems in theprior art noted above and to provide an improved method of making afiber optic array for use in scanning devices.

In accordance with one aspect of the invention, there is provided a amethod of making a fiber optic array, the method comprising the stepsof: forming a plurality of sets of grooves in a substrate, each set ofgrooves being separated by an area on said substrate; inserting thejackets of optical fibers in one set of grooves and fixing the jacketstherein; inserting exposed cladding of the fibers is the remaining setsof grooves and fixing the fibers in the remaining sets of grooves.

The fiber optic array of the present invention includes an array ofoptical fibers supported in grooves formed in a substrate. The groovesare arranged in aligned sets, and each set of grooves is separated byplanar areas on the substrate which are coplanar with the bottoms of thegrooves. At an input end of the substrate is a set of grooves which aresized to receive the jackets of the optical fibers, and sets of grooves,which are adapted to receive the cladding of the fibers, are spacedalong the substrate to an output end of the substrate. The pitch of thegrooves in successive sets decreases as the fibers approach the outputend of the substrate in order to draw the fibers together at the outputend. In one embodiment of the present invention, sets of closely spacedV-shaped grooves are etched in a silicon substrate, and an optical fiberhaving a cladding portion of reduced diameter is inserted in each of thegrooves to form a linear array of fibers. An adhesive is used to securethe fibers in the grooves.

A principal advantage of the array of the present invention is that theoptical fibers can be supported on the substrate in close proximity toeach other in a manner which does not damage the fibers or affect theperformance of the fibers. As a result of supporting the fibers in setsof grooves separated by planar areas, the fibers can be placed ingrooves of increasingly smaller pitch without being forced through sharpturns which would damage the fibers. Further, the planar areas betweenthe grooves facilitate the cementing of the fibers in the grooves.

A further advantage of the array of the present invention is that a veryhigh packing density is obtained as a result of using fibers in whichthe cladding diameter at one end is relatively small. Moreover, thefibers are constructed such that they have the strength and rigidity tobe handled in the same manner as fibers which do not have ends ofreduced diameter. Outputs from the closely-spaced andindependently-addressable fibers can be imaged onto a receiving mediumthrough a single train of optics with diffraction-limited performance.

Other features and advantages will become apparent with reference to thefollowing Description of the Preferred Embodiments when read in light ofthe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a fiber optic array which has been madein accordance with the present invention;

FIG. 2 is an enlarged elevational view of a portion of one end of thearray showing the spacing of the grooves and the location of an opticalfiber in a groove;

FIG. 3 is a plan view of the substrate used in the present invention;

FIG. 4 is a view of a fiber which has been prepared for etching;

FIG. 5 is a view of a fiber after etching;

FIGS. 6A-6C are sectional views taken along the lines 6A--6A, 6B--6B,and 6C--6C, respectively, in FIG. 3; and

FIGS. 7-13 are views showing the sequence of steps for forming groovesin the substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, there is shown a fiber optic array 10 whichhas been made in accordance with the present invention. Fiber opticarray 10 comprises three optical fibers 12 which are supported on asubstrate 16. The full length of only one of the fibers 12 is shown inorder to more clearly illustrate various features of the invention; itwill be understood, however, that each of the fibers 12 is identical andextends the full length of substrate 16. Each of the fibers 12 isconnected by means of an optical fiber connector 18 to another opticalfiber 20. Optical fiber connector 18 can be of the type shown incommonly-assigned U.S. Pat. No. 4,723,830, entitled Optical FiberConnectors, issued Feb. 9, 1988. Each optical fiber 20 is connected to alight source 22, such as a diode laser or a light-emitting diode. Eachlight source 22 in array 10 can be modulated according to an informationsignal in a well-known manner. Array 10 can be used to recordinformation on a light-sensitive medium (not shown) such as aphotoconductor or photographic film. Array 10 can also be used as anoptical scanner (not shown) in which light reflected from an object istransmitted through the optical fibers to photosensitive elements.

As shown in FIGS. 4 and 5, each of the optical fibers 12 includes ajacket 30, a cladding 32, and a core 34. Jacket 30 has been removed froman end 31 of the fiber 12 to expose the cladding 32, and in a claddingportion 33 (FIG. 5), the diameter of the cladding is substantiallyreduced so that the portions 33 can be more closely spaced relative toeach other. A tapered cladding portion 36 is formed at one end ofportion 33. A fiber suitable for use in the present invention is aspecial single-mode fiber made by Corning Glass Works, Fiber No.56704121,KH1. This fiber is made of silica with germanium doping in thecore 34. The jacket 30 has a diameter of 250 μm, the cladding 32 has adiameter of 125 μm, and the core 34 has a diameter of 4 μm.

Although the invention herein is described using optical fibers havingends of reduced diameter, the array of the present invention can alsoutilize fibers in which the diameter of the cladding is the samethroughout the length of the fiber.

As shown in FIG. 1, fibers 12 extend from an input end 15 of array 10which supports jackets 30 of the fibers to an output end 17 of thearray, and the fibers 12 are closest together at end 17. Fibers 12 aremounted in sets of grooves 28a-28g which are formed in substrate 16 andare separated by planar areas 29a-29f in the substrate 16. Grooves28a-28g in each set are separated by lands 25a-25g, respectively.Grooves 28a are generally rectangular in cross section, as shown in FIG.6A, and grooves 28b-28g are generally V-shaped in cross section as shownin FIG. 6B. In a preferred embodiment, the areas 29a-29f are coplanarwith the groove tips or bottoms of adjacent grooves; one of the groovetips is shown at 27b in FIG. 6B.

As shown in FIG. 2, a glass plate 21 extends over the fibers 12 ingrooves 28g, and a similar plate (not shown) extends over the fibers 12at each of the other sets of grooves 28a-28f. These glass plates arevery essential in the fabrication of array 10 as will be explainedhereinafter. The glass plates 21 have been omitted in FIG. 1 in order tomore clearly show elements of the present invention. At each set ofgrooves 28a-28g, a plate 21 extends over the fibers 12 and the grooves,and as shown in FIG. 2, an epoxy 23 fills the void areas between theplate 21, the substrate 16 and the fibers 12.

Grooves 28a-28g are arranged to space fibers 12 progressively closertogether in the direction of output end 17 of the array 10. Theprogressively closer spacing is accomplished by decreasing the pitch p(FIG. 6B) of the grooves in successive sets of grooves 28a-28g. As shownin FIG. 6B, the pitch p of the grooves includes the width W of thegrooves and width S of the lands 25a-25g. The pitch p can be changed bychanging the width S of the land, or by changing the width W of thegrooves, or by changing both S and W.

Grooves 28a are sized to receive the jackets 30 of fibers 12. Grooves28b-28f are adapted to receive the cladding 32 of the fibers 12, andgrooves 28g are adapted to receive the cladding portions 33 of reduceddiameter. It is important that the jackets 30 remain on fibers 12 onsubstrate 16 for a certain length of the fibers to lend stability toarray 10. In some embodiments, it may be desirable for the claddingportion 33 to extend into other sets of the grooves 28b-28f. Thedimensions of the grooves 28a-28g will depend on the size and type offiber used in array 10. For a single mode fiber of the type describedabove, obtainable from Corning Glass Works, the grooves 28a are fromabout 255 to about 280 μm wide, and grooves 28b-28f are about 155 μmwide; the dimensions of grooves 28g for various sizes of claddingportion 33 are given in Table I which follows. Starting with grooves 28band progressing through successive sets of to grooves 28g, the grooves28b-28g are separated by lands 25b-25g having progressively thinnerwidths S in order to draw the fibers 12 closer together at end 17 of thearray 10. For example, the widths S of the lands 25a-25g can be,respectively, 25, 100, 76, 53, 29, 5, and 4 μm.

In one illustrative example of the present invention, the length of thesubstrate 16 is about 75 mm, the width of the substrate is about 25 mm,and the substrate is about 525 μm thick. The length of the grooves 28ais about 10 mm, the length of grooves 28b-28f is about 2 mm, and thelength of grooves 28g is about 25 mm. The length of planar areas 29a-29eis about 4 mm, and the length of planar area 29f is about 10 mm.

The dimensions of grooves 28g and of portions 33 of fibers 12 areindicated in FIG. 2 where W is the width of a groove 28g, β is the halfangle of a groove 28g, S is the width of the land 25a between adjacentgrooves 28g, and P is the channel separation, that is the distancebetween the grooves 28g at the bottoms 27g thereof. D is the diameter ofcladding portion 33 of a fiber 12. The angle β is about 35° when thegrooves 28g are etched in silicon; however, other angles can be used.The dimensions of grooves 28g for various diameters D are shown in TableI.

                  TABLE I                                                         ______________________________________                                        D (μm)                                                                              W (μm)      S (μm)                                                                             P (μm)                                     ______________________________________                                        10       12             4       16                                            20       24             4       28                                            30       36             4       40                                            40       49             4       53                                            50       61             4       65                                            100      146            4       150                                           ______________________________________                                    

FIBER ETCHING

In FIG. 4, there is shown a fiber 12 in which the jacket 30 has beenremoved from a portion thereof in preparation for an etching step. Fiber12 is etched in hydrofluoric acid (HF) in order to reduce the diameterof cladding 32 from, for example, 125 μm to a diameter as small as 10μm. The nominal etching rate at room temperature is 1.00 μm/min. Thediameter of portion 33 must be large enough so there is no coupling oflight between adjacent fibers. For best results, the diameter of thecladding in portion 33 should be at least 10 μm.

A fiber 12, after etching, is shown in FIG. 5. During the etchingprocess, the ends of fibers 12 are placed in the hydrofluoric acid, andthe tapered portion 36 is formed above the surface of the HF acid as aresult of capillary action. It has been found that the tapered portion36 has a beneficial effect in that the bending strength of the fiber 12is substantially greater than it would be if there were a sharp changefrom the normal diameter of cladding 32 to the cladding portion 33 ofreduced diameter. The core 34 of fiber 12 is etched faster than thecladding 34; as a result, a concave tip (not shown) is formed on core 34which must be removed by polishing after the etching step.

SUBSTRATE PREPARATION

The substrate 16 is preferably formed from a silicon wafer (not shown)by photolithographic means. With reference to FIGS. 7-13, there areshown the steps for forming the grooves 28b-28g and planar areas 29a-29fin the wafer. As shown in FIG. 7, a layer 50 of Si₃ N₄ is formed on asilicon wafer 52, and as shown in FIG. 8, a negative photoresist 53 isformed over the Si₃ N₄ layer 50. The negative photoresist 53 can be, forexample, photopolymerizable plastic such as negative photoresist, type747, supplied by KTI Chemical, Inc. Photoresist 53 is exposed withultraviolet light through a mask 54 (FIG. 9). The ultraviolet light canbe provided by mercury vapor lamps. In the next step, the photoresist isdeveloped, using, for example, Projection Developer obtainable from KTIChemical, Inc., to remove the unexposed portions of photoresist, asshown in FIG. 10. Next, the Si₃ N₄ layer 50 is etched, using plasmaetching, and the photoresist 53 is stripped, as shown in FIG. 11. Theexposed portions of wafer 52 are then etched to form the grooves28a-28g, indicated at 28 in FIG. 12, and the planar areas 29a-29f ofsubstrate 16. After the grooves and planar areas are formed, the Si₃ N₄layer 50 is removed by plasma etching, and the wafer 52 appears as shownin FIG. 13. It has been found that removal of the Si₃ N₄ layer 50 is avery important step, since the adhesive used to cement the fibers 12 inthe grooves does not adhere well to the Si₃ N₄ under all conditions. Thewafer 52 is cleaned before and after etching using acetone and deionizedwater, and the wafer is then blown dry with nitrogen.

The process described above is used to form grooves 28b-28g and planarareas 29a-29f in substrate 16. Grooves 28a at end 15 of the substrate 16are formed last, and they are formed with a diamond blade. Grooves 28aare adapted to receive the jackets 30 of the fibers 12. In oneillustrative example, the grooves 28a, as shown in FIG. 6A are generallyrectangular in cross section; and the width W is between about 255 andabout 280 μm, the depth d is about 150 μm, and the width S of the lands25a is about 25 μm.

Great care must be used in the mounting of fibers 12 on substrate 16 duethe fragility and relatively small size of the fibers 12. In a firststep, the fibers are arranged parallel to each other in a holder (notshown) having channels to receive the jackets 30. The jackets 30 of thealigned fibers 30 are then inserted in the grooves 28a on substrate 16.A glass plate, for example, a plate such as plate 21 shown in FIG. 2, isplaced over the jackets 30 in grooves 28a. The glass plate can be about150 μm thick and should be of a size to extend over grooves 28a. Theglass plate is held in place by a needle in a micropositioner, model221, obtainable from Rucker and Kolls. An ultraviolet light curableepoxy is introduced between the glass plate and substrate 16, and theepoxy is drawn into grooves 28a around the jackets 30 of fibers 12 bymeans of capillary action. The epoxy is cured by ultraviolet light whichis directed onto epoxy through the glass plate. The cladding 32 offibers 12 is then eased into grooves 28b-28g of progressively finerpitch, and the fibers 12 are cemented in place in each set of grooves28b-28g by means of a glass plate and UV curable epoxy as describedpreviously; that is, the glass plate is placed directly over the fibers12 in the grooves, held in position by a micropositioner, and UV curableepoxy is introduced around the fibers 12 in the grooves by means ofcapillary action.

The planar areas 29a-29f provide an area in which the fibers 12 aredrawn closer together between the sets of generally parallel grooves28b-28g of progressively smaller pitch. The planar areas 29a-29f arealso important in the assembly steps just described in that they providea means for viewing the fibers to determine if the fibers are actuallyaligned, since it is very difficult to see the fibers in the sets ofgrooves 28b-28g.

When the fibers 12 have been cemented in grooves 28a-28g, a portion ofthe array facet 19 is removed by means of a dicing saw (not shown), andthe facet 19 is then polished. The dicing saw can be a resin impregnateddiamond blade, and the dicing step can be accomplished by cuttingthrough the glass plate covering cladding portions 33, optical fibers 12and the substrate 16 simultaneously.

An important element of the present invention is the adhesive used tocement the fibers 12 to the substrate 16. The adhesive must have lowviscosity for good capillary action and a lower refractive index thanthat of the cladding 32 to minimize radiation loss and cross talkbetween fibers 12. After the adhesive has cured, there should be lowstress on the fiber to minimize micro-bending loss, and the adhesiveshould have adequate hardness to insure a polished surface of highquality. A suitable adhesive is Norland 61 adhesive which can beobtained from the Norland Co. However, a preferred adhesive is Lamdek UV Adhesive, Catalog No. 177 6921, which can be obtained from DymaxEngineering Adhesives, a division of the American Chemical andEngineering Co., Torrington, CT.

In an illustrative example of the present invention, an array 10 wasformed from single mode fibers having a core diameter of about 4 μm anda cladding diameter of about 125 μm. The portions 33 were etched to adiameter of about 15 μm, the length of portions 33 was about 25 mm, andthe channel separation (P) between grooves 28g on substrate 16 was about28 μm. The fibers 12 were mounted on a substrate 16 as shown in FIG. 1.The array thus formed was evaluated for taper/packaging loss, cross-talklevel, and the output-beam quality. The taper/packaging loss is anindication of the radiation loss due to fiber taper, to etched cladding,and to packaging stress in comparison to the radiation loss due to thesefactors from a single mode fiber which has not been etched. It was foundthat the propagation characteristics of the optical field were changedvery little due to the etching and packaging processes. Thus, asubstantial increase in packing density has been achieved in the arrayof the present invention while maintaining the necessary efficiency ofthe fibers.

It will be apparent that the number and spacing of fibers 12 can bechanged to meet the needs of a particular application. An array (notshown) could extend the full length of a recording medium, or arecording head (not shown) could include a plurality of arrays arrangedside-by-side or in parallel rows. An array 10 can also be used toproduce various print formats, as shown, for example, in acommonly-assigned patent application entitled Method of Scanning, U.S.Ser. No. 254,745, filed in the name of D. Haas on even date herewith.

The invention has been described in detail with particular reference toa preferred embodiment thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

We claim:
 1. A method of making a fiber optic array, said methodcomprising the steps of:forming a plurality of sets of grooves in asubstrate, each set of grooves being separated by an area on saidsubstrate; inserting the jackets of optical fibers in one set of groovesand fixing the jackets therein; inserting exposed cladding of saidfibers in the remaining sets of grooves and fixing the fibers in saidremaining sets of grooves.
 2. A method, as defined in claim 1, whereincertain sets of said grooves are formed by photolithographic techniques.3. A method, as defined in claim 1, wherein said areas are planar areaswhich are generally coplanar with bottoms of said grooves.
 4. A method,as defined in claim 3, wherein said planar areas are formed byphotolithographic techniques.
 5. A method of making a fiber optic array,as defined in claim 1, wherein said fibers are fixed in said grooves bya cement curable by ultraviolet light.
 6. A method of making a fiberarray, as defined in claim 1, wherein said one set of grooves are formedin one end of the substrate by a saw.
 7. A method of making a fiberoptic array, said method comprising the steps of:forming sets of groovesand planar areas between said sets of grooves by etching through a maskof Si₃ N₄ ; removing said mask by plasma etching; placing optical fibersin said grooves such that the ends of said fibers form a linear array;and cementing said fibers in said grooves.
 8. A method of making a fiberarray, as defined in claim 7, wherein said fibers are cemented in saidgrooves using an adhesive curable by ultraviolet light.